Recording method for phase-change recording medium

ABSTRACT

In a recording method for a rewritable optical recording medium, immediately before a prospective leading pulse that is a time section where recording light of write power Pw irradiates a phase-change recording layer for the first time as a front low-power energy irradiating step, light of bias power Pb irradiates the phase-change recording layer for a first set time length yT as a preceding low-power energy irradiation step. And immediately after the leading pulse, the light of bias power Pb irradiates the phase-change recording layer for a set time length xT as a succeeding low-power energy irradiation step. The relation between x and y satisfies: 0.95 ≦x+0.7*y≦2.5. The high-power energybeam irradiates the recording medium in such a manner that a period of irradiation for pulses subsequent to the leading pulse is in a range of from 0.5T to 2.5T. The result is that it is possible to facilitate forming/erasing amorphous marks even in a phase-change recording medium devoid of a reflective layer, without any risk of narrowing the range of effective crystallization speeds.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a phase-change recording methodsuitable for recording on a phase-change recording medium, which can bein a multiplayer structure and is large in transmission factor(transmittance) like an optical disc enabling high-density recording, byutilizing a rewritable recording phase-change medium.

[0003] 2. Description of Related Art

[0004] Nowadays, as information amount growingly expands, demands forrecording mediums which can record/retrieve a large capacity of data athigh density and high speed have been on the rise. Therefore opticaldiscs have been expected to meet these growing demands. Optical discsare divided into two types: one type enabling recording for only onetime, and the other type enabling recoding/erasing for many times. Therewritable optical discs are exemplified by a magneto-optical recordingmedium utilizing a magneto-optical effect, and a phase-change mediumutilizing change in reflectance with reversible change in crystallinestate.

[0005] This phase-change recording medium can make recording/erasingonly by modifying laser light power (output) without outer magneticfield and enables downsizing a recording/retrieving apparatus. Further,if the phase-change recording medium is used, it is possible to realizerecording/retrieving information in high density by a short-wavelengthlight source without changing the material of a recording layer inparticular from a medium that is recordable/erasable by light powerabout 800 nm in wavelength, which is most popular in the art.

[0006] Thin films of chalcogen alloy are used for the recording layermaterial of many of commercially available phase-change mediums. Thischalcogen alloy is exemplified by GeSbTe alloy, InSbTe alloy, GeSnTealloy, and AgInSbTe alloy. In the currently practical recording methodfor a rewritable phase-change recording medium, the recording layertakes a crystalline state as unrecorded/erased state, and an amorphousbit is formed for recording. An amorphous bit is formed by heating therecording layer up to a temperature higher than a melting point and thenrapidly cooling down the recording layer.

[0007] In order to prevent possible vapor and deformation, which mightoccur due to the heating of the recording layer, the recording layer isordinarily sandwiched a set of upper and lower dielectric protectivelayers which are resistant to heat and chemically stable. Further, ingeneral, a metallic reflective layer is placed on the sandwich structureto provide a quadri-layer structure so that heat dispersion isfacilitated and amorphous marks are formed stably.

[0008] This metallic reflective layer serves to escape heat generatedwhen the recording layer is irradiated by a recording laser light beam(hereinafter also called “light beam”). Namely, if an amorphoussubstance is used in a phase-change medium, the recording layer islocally melted by the light beam, and then the resulting recording layeris rapidly cooled to form an amorphous mark. In the presence ofinadequate radiation, this amorphous mark cannot be formed neatly asintended; consequently the metallic reflective layer is required.

[0009] In order to form the amorphous mark stably, the divided pulsemethod has been customary to divide a mark-forming laser pulse. On manyoccasions, assuming that a reference clock period is T, a pulse sequenceof the period T is irradiated according to the mark length. At thattime, to make the temperature distribution in the mark uniformly, thetime length of the leading pulse (the first pulse) is set to larger thanthat of the second and subsequent pulses.

[0010] The divided pulse method is a method of forming an amorphous markof a time length nT by alternately irradiating the phase-changerecording medium with a write power Pw having a relative high powervalue and a bias power Pb having a relatively low power value. Here n isa natural number equal to or larger than 4).

[0011] Specifically, of the output pattern (pulse pattern) of the lightbeam, a writing pulse to be output at high power is divided into aplurality of pulses, and an off-pulse to be output at low power isdivided into a plurality of pulses; these high and low powerirradiations are alternately repeated. In the conventional divided pulsemethod, for every pulse, the time length of the divided writing pulse tobe output at high power and the time length of the divided off-pulse arenearly 0.5T.

[0012] The pulse patter when the amorphous mark is formed is disclosedin the following publications 1, 2 and 3. Publication 1 is “TheFeasibility of High Data Rate 4.7 GB Media with Ag-In-Sub-Te PhaseChange Material”, (Collection of Theses presented in 10th Symposium ofPhase Change Media Society 1998) disclosing a technology relating to thepulse pattern.

[0013] Publication 2 is “Rewritable Dual-Layer Phase-change OpticalDisk”, Jpn. J. Appl. Phys. Vo. 38(1999) pp. 1679-1686) disclosing atechnology relating to an optical disk in the form of a rewritablephase-change medium having a multiplayer structure. SpecificallyPublication 2 describes a rewritable pulse pattern.

[0014] Publication 3 is Japanese Laid-Open Publication No. Hei 3-185628(U.S. Pat. No. 5,109,373) disclosing a technology relating to a methodand apparatus for recording signals on an optical information recordingmedium, such as an optical disk, for recording/retrieving opticalinformation at high speed and in high density using a laser light beam.Specifically, Publication 3 describes a value satisfying a repeatingperiod τ in an intermediate pulse sequence.

[0015] In the meantime, for erasing (crystallizing), the recording layeris heated up to a temperature higher than a crystallization point of therecording layer and lower than a melting point of the recording layer.In this case, the dielectric protective layer serves a heat storagelayer to keep the recording layer at a high temperature enough forcrystallization.

[0016] Further, in a 1-beam overwritable phase-change medium, theabove-mentioned erasure/rewrite processes are carried out only bymodifying a single focused light beam. This technology is disclosed inPublication 4 (Jpn. J. Appl. Phys. 26 (1987), suppl. 25-4, pp. 61-66).Furthermore, by using 1-beam overwritable phase-change medium, the layerstructure of a recording medium and the circuit structure of a recordingdrive apparatus would be simple. Therefore a system using a 1-beamoverwritable phase-change medium is watched with a keen interest forinexpensiveness, high density and large capacity.

[0017] Recently, attempts have been made to increase the number oflayers of a recording medium to a much higher density. An attempt toincrease the recording density is to manufacture two or more recordingmedium parts spaced from each other by a distance larger than the focusdepth of an optical system being used. In this attempt, the recordingmedium parts except the farthest recording medium part, as viewed fromthe substrate where the laser light comes in, respectively require ahigh transmission factor of 30% or more to permit the laser light topass.

[0018] Consequently, in order to permit the laser light, it is essentialthat basically no metallic reflective layer is used, or a metallicreflector has such a small thickness as to permit adequate light topass.

[0019] Yet in the recording medium devoid of a metallic reflective layeror having a thin metallic reflective layer, since only inadequate heatradiation effect can be achieved, re-crystallization would tend to occurwhen an amorphous mark is formed, so that an amorphous mark neat asintended is difficult to form.

[0020] There could be another attempt to prevent re-crystallization bymodifying the composition of the recording layer at least to make thecrystallization speed slow. But because of the slow crystallizationspeed, an amorphous mark to be irradiated by the erasure power(hereinafter called “erasure power irradiation section”) after havingbeen formed can be crystallized only inadequately so that the amorphousmark cannot be erased.

[0021] Namely, when making recording on a phase-change medium that isdevoid of a metallic reflective layer or has a thin metallic layer, itis difficult to prevent re-crystallization of the mark during recordingwhile keeping adequate erasure ratio at the erasure power irradiationsection, thus narrowing the range of effective crystallization speedsfor a rewritable recording medium.

SUMMARY OF THE INVENTION

[0022] With the foregoing problems in view, it is an object of thepresent invention to provide a recording method for a rewritablephase-change recording medium which method facilitates forming/erasingamorphous marks even if a reflective layer of the recording medium isonly a limited thickness or void, without any risk of restricting therange of effective crystallization speeds.

[0023] In order to attain the above object, according to a genericfeature of the present invention, there is provided a recording methodfor a phase-change recording medium having a phase-change recordinglayer in which amorphous marks each having a time length nT (T is thedata reference clock period, and n is a natural number equal to orlarger than 4) are formed by alternately irradiating the recordingmedium at least with a high-power energy beam having a relatively highpower value and a low-power energy beam having a relatively low powervalue, said method comprising the steps of: (a-1) irradiating thelow-power energy beam to the recording medium for a first set time yT (yis a natural number larger than or equal to 0) as a preceding low-powerpulse irradiation step immediately before a prospective leading pulsethat is a time section where the high-power beam is irradiated for thefirst time, and (a-2) irradiating the low-power energy beam to therecording medium for a second set time xT (x is a natural number largerthan 0) as a succeeding low-power pulse irradiation step immediatelyafter said prospective leading pulse, x and y having a relationexpressed by the following formula

0.95≦x+0.7*y≦2.5

[0024] where * is an arithmetic symbol representing a multiplication;and (b) irradiating the high-power energy beam to the recording mediumin such a manner that a period of irradiation for pulses subsequent tothe leading pulse is in a range of from 0.5T to 1.5T.

[0025] According to this recording method of the present invention, itis possible to prevent re-crystallization of the marks during recording,which would have been encountered with a small-thickness phase-changerecording medium, even if the phase-change recording medium is devoid ofany metallic reflective layer. And since the conventional pulses exceptfor the leading end portion of a divided pulse signal can be utilizedwithout any special reconstruction, it is possible to facilitate circuitdesigning.

[0026] Further, it is possible to prevent re-crystallization of themarks during recording, with keeping an adequate erasure ratio at theerasure power irradiation section, even if information in terms ofdifferent mark lengths is recorded on a phase-change recording mediumdevoid of a metallic reflective layer or having a thin metallicreflective layer, thus eliminating the conventional problem thatrestricts the range of effective crystallization speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a laser pulse waveform diagram illustrating arepresentative pulse pattern to be used in a recording method, for anoptical phase-change recording medium, according to one embodiment ofthe present invention;

[0028]FIG. 2 is a diagram illustrating pulse lengths in the pulsepattern of FIG. 1;

[0029]FIG. 3 is a diagram schematically showing a multi-layer structureof a phase-change recording medium;

[0030]FIG. 4 is a graph illustrating transition of the erasure ratiowith respect to the erasure power in the present embodiment;

[0031]FIG. 5 is a graph illustrating a relation between x of asucceeding low-power pulse irradiation section and the jitter in thepresent embodiment;

[0032]FIG. 6 is a graph illustrating a relation between y of thepreceding low-power pulse irradiation section and the jitter in thepresent embodiment; and

[0033]FIG. 7 is a graph illustrating a relation between the value ofx+0.7*y and the jitter in the present embodiment.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0034] A preferred embodiment of the present invention will now bedescribed with reference to the accompanying drawings.

[0035] The present inventor(s) have discovered that, in a phase-changerecording medium of the type in which the cooling effect is small,re-crystallization of an amorphous mark tends to progress from theleading edge of the mark to the trailing edge of the mark duringrecording. The inventor(s) have also discovered that rapid cooling ofthe leading mark edge, i.e. a start portion of the mark being formed,prevents the mark from being re-crystallized, thus reaching the presentinvention.

[0036] According to the present invention, because an off-pulse isinserted contiguously to a writing pulse, which forms the leading edgeof an amorphous mark, in a pattern of successive pulses for forming anamorphous mark during the recording using a divided pulse method, it ispossible to cool the leading edge of the mark rapidly so thatre-crystallization can be prevented.

[0037] Namely, in a pattern of successive pulses, only the off-pulse,the writing pulse forming the leading edge of the mark, are modifiedfrom that in the conventional method to prevent re-crystallization.According to the present invention, since the divided pulse methodemploying the conventional technology can be used for the pulse patternexcept the divided pulse, it is possible to limit the number ofmodifications to a minimum so that a circuit reflecting themodifications is easy to design.

[0038] The principles of the present invention is particularly usefulwhen applied to an optical disc having a phase-change recording layer.Generally an optical disc has a spiral groove or concentric grooves, anda land or lands defined between the grooves; the inter-groove landserves or serve as a recording track for recording and retrieving. In aCD-RW or a DVD-RW, which are each another type of phase-change opticaldisc, the inside of the groove serves as a recording track whileamorphous marks are formed in a crystalline recording layer.

[0039] This type optical disc has a quadri-layer structure composed of aprotective layer, a phase-change recording layer, a protective layer,and a reflective layer, which are disposed one over another reversely inthe described order on the substrate. In still another optical recordingmedium having two or more recording layers for the purpose of increasingthe capacity, a protective layer, a phase-change recording layer, aprotective layer, a resin layer, a protective layer, a phase-changerecording layer, a protective layer, and a reflective layer are disposedone over another reversely in the described order on the substrate. Inan alternative, two optical discs each having this structure areattached to one another. In either type optical disc, an energy beamenters the recording layer(s) from the substrate side forrecording/retrieving. In another alternative, with the various layers onsubstrate being arranged exactly in the reverse order, an energy beamenters the recording layer(s) from the side opposite to the substratefor recording/retrieving.

[0040] The foregoing results are particularly remarkable when thepresent invention is applied to a recording medium that is void of areflective layer disposed contiguously to the recording layer with orwithout the medium of a protective layer, or a recording medium that hasa thin reflective layer of 30 nm or smaller in thickness contiguously tothe recording layer with or without the medium of a protective layer.

[0041] In other words, in a conventional recording medium having anordinary-thickness reflective layer, which would tend to encounterre-crystallization, rapid cooling of the leading edge of the markaccording to the present invention is relatively less effective toprevent re-crystallization.

[0042] The process of re-crystallization, in which the formed amorphousarea is progressively re-crystallized from the edge of the amorphousarea and of the crystalline area around the amorphous area edge as theamorphous mark is formed, is typically exemplified by the following.

[0043] It is supposed from the studies of the present inventor(s) that,in the case of a recording medium whose cooling effect is small,re-crystallization tends to occur at the leading edge of the mark,particularly from the transversely (perpendicularly to the trackingdirection) central portion of the mark edge, and progresses from thecentral portion toward the trailing edge of the mark. On the other hand,it is presumed that, in the case of a recoding medium whose coolingeffect is large, re-crystallization tends progresses from the transverseedge of the mark toward the center.

[0044] Because the present invention retards re-crystallization byrapidly cooling the leading edge of the mark, it is possible to retardsthe progress of re-crystallization from-the leading edge of the marktoward the leading edge of the mark when the present invention isapplied to the recording medium whose cooling effect is small; thecooling effect is large. Therefore, in the case of a recording mediumthat is void of a reflective layer disposed contiguously to therecording layer with or without the medium of a protective layer, or arecording medium that has a thin reflective layer of 30 nm or less inthickness disposed contiguously to the recording layer with or withoutthe medium of the protective layer, the cooling effect is remarkablylarge.

[0045] For example, if the phase-change recording medium is a recordingmedium that has two or more phase-change recording layers with anotherlayer sandwiched therebetween, because it is essential to expose alsothe farther phase-change recording layer to the recording/retrievingenergy beam, a thick reflective layer cannot be located in the travelingpath of the energy beam. As a consequence, the nearer phase-changerecording layer is limited to a recording medium that is void of areflective layer disposed contiguously to with or without the medium ofa protective layer, or a recording medium that has a very thinreflective layer that is 30 nm or smaller in thickness. The presentinvention is particularly remarkable in cooling effect when recording ismade in the nearer phase-change recording layer with respect to therecording/retrieving energy beam.

[0046] For preventing re-crystallization of the leading edge of themark, according to the present invention, it is essential to rapidlycooling the leading mark edge. First of all, a pulse patter will now bedescribed with reference to FIGS. 1 and 2.

[0047]FIG. 1 is a laser pulse waveform diagram illustrating arepresentative pulse pattern to be used in the recording methodaccording to the present invention. The horizontal coordinate representsthe time, and the vertical coordinate represents the laser power; Pw isthe write power, Pe is the erasure power, Pb is the bias power, and Pris the retrieve power. Basically, the write power Pw and the bias powerPb are used for forming the mark, and the erasure power Pe is used forerasing. And the retrieve power Pr is used for retrieving the recordedinformation.

[0048] In the following description, a pulse to be output by high power(write power Pw) is called “writing pulse” or “recording pulse”, and atime section during which a recording pulse is irradiated with the writepower Pw is called “recording pulse section” or “recording powerirradiation section”. The latter section corresponds to a high-powerlaser irradiation section.

[0049] Further, a pulse to be output by low power (bias power Pb) iscalled “off-pulse”, and a time section during which the off-pulseirradiates is called “off-pulse section” or “bias power irradiationsection”. The latter section corresponds to a low-power laserirradiation section.

[0050]FIG. 1 illustrates how to form a mark having a time length 10T (Tis a reference clock period). The pulse pattern of FIG. 1 is dividedinto 9 writing pulses designated by reference numbers 1, 2, . . . , 9.In a general sense, assuming that a mark having a time length nT isformed, the number (m) of write pulses is any one of n, n−1 and n−2. Inthe illustrated example of FIG. 1, m=n−1.

[0051] Each of these 9 writing pulses 1 through 9 has a power valueequal to a respective write power Pw, and each of off-pulses precedingand succeeding the individual writing pulse has a power value equal to arespective bias power Pb. Further, each of radiation power values fortime sections except the writing pulses and off-pulses is equal to anerasure power Pe.

[0052] Ordinarily the write power Pw is preferably 20 mW or lower, morepreferably 14 mW or lower. It is a common knowledge that the lower thelaser power to write, the more the recording medium is regarded aspreferable; this is true because using a low output of laser doessuffice. Practically, however, the write power Pw is preferably 8 mW orhigher. Because writing by a too low power means tending to becomeinferior when retrieving.

[0053] For the erasure power Pe, a value is selected such that an oldamorphous mark to be erased is adequately crystallized. Ordinarily theerasure power Pe is within a range of from 30% to 70% of the write powerPw.

[0054] The retrieve power Pr is the power of an energy beam to beirradiated when retrieving the recorded information; for the retrievepower Pr, a low value is selected, ordinarily within a range of from 0.5to 1.0 mW.

[0055] For the bias power Pb, a value is selected such that therecording layer heated by the write power Pw is rapidly cooled to forman amorphous mark. To increase the cooling rate of the recording layer,the bias power Pb is preferable low. The ratio of the bias power Pb andthe erasure power Pe is ordinarily Pb/Pe ≦0.5, preferably Pb/Pe≦0.3.With a view to tracking ability, the bias power Pb is close or equal tothe value of the retrieve power Pr.

[0056] Consequently, as a generic feature, the recording method of thepresent invention is a recording method for a phase-change recordingmedium having a phase-change recording layer in which amorphous markseach having a time length nT (T is the data reference clock period, andn is a natural number equal to or larger than 4) are formed byalternately irradiating the recording medium at least with a high-powerenergy beam having a relatively high power value and a low-power energybeam having a relatively low power value, comprising the followingsteps:

[0057] a preceding low-power pulse irradiation step, immediatelypreceding a prospective leading pulse that is a time section where thehigh-power beam is irradiated for the first time, in which the low-powerenergy beam is irradiated to the recording medium for a first set timeyT (y is a natural number larger than or equal to 0; and

[0058] a succeeding low-power pulse irradiation step, immediatelysucceeding the prospective leading pulse, in which the low-power energybeam is irradiated to the recording medium for a second set time xT (xis a natural number larger than 0).

[0059] And x and y has a relation expressed by the formula0.95≦x+0.7*y≦2.5 where * is an arithmetic symbol representing amultiplication.

[0060] Namely in the present recording method, each of off-pulsesections preceding and succeeding the leading pulse (writing pulse 1) ofFIG. 1 has a large length (time length) as compared to that used in theconventional divided pulse method.

[0061] In the pulse pattern as depicted by FIG. 1, for the purpose ofrapidly cooling the leading edge of an amorphous mark, a low-poweroff-pulse having a time length xT is located between the leading pulse(writing pulse 1) of a divided pulse for forming the mark and the secondpulse (writing pulse 2) where x is a natural number larger than 0.

[0062] The studies made by the present inventor(s) indicate that as longas at least the condition as of the laser power is within an ordinaryrange, the leading edge of the mark tends to be cooled if the timelength xT of the low-power laser irradiation section (off-pulse) is setremarkably long as compared to that in the conventional method, thusobtaining an excellent recording characteristic. The off-pulse having atime length xT succeeding the writing pulse 1 is hereinafter also called“xT pulse”.

[0063] It is also effective to locate an off-pulse having a time lengthyT immediately preceding the leading pulse where y is a natural numberlarger than or equal to 0. The off-pulse having a time length yTpreceding the writing pulse 1 is hereinafter also called “yT pulse”.

[0064] If the sum of x and 0.7*y is too small, an intended coolingeffect for the leading edge of the mark cannot be achieved. Consequentlythat value is 0.95 or larger, preferably 1.3 or larger. Otherwise if thesum of x and 0.7*y is too large, there is a danger that an old amorphousmark might be left erased, and/or the amorphous local portion by theleading pulse and the amorphous local portion succeeding the writingpulse might would optically separated. Consequently that value is 2.5 orsmaller, preferably 2.0 or smaller.

[0065] Further, the value of y may be close (small) to or equal to 0. Ify is 0, the leading pulse being a time section during which the writepower Pw is irradiated for the first time is irradiated for apredetermined time (leading pulse irradiation step). Then in thesucceeding low-power pulse irradiation step immediately succeeding theleading pulse irradiation step, the bias power Pb is irradiated for asecond set time xT. x may be 0.95≦−x≦−2.5. Namely, increasing xT pulsein time length without providing yT pulse suffices to obtain theintended cooling effect.

[0066] However, since it is essential to increase xT pulse in timelength for achieving the same cooling effect without providing yT, thewriting pulse 1 would be located forwardly, which would be a possiblecause for increasing the length of the whole mark. Consequently it ispreferable and/or effective to combine xT pulse and yT pulse.

[0067] If the value x is excessively small, it would be difficult toretard re-crystallization of the leading edge of the mark. Consequently,x is ordinarily equal to or larger 0.1, preferably equal to or largerthan 0.3. Otherwise if the value x is excessively large, there is adanger that an old amorphous mark might be left erased, and/or theamorphous local portion by the leading pulse and the amorphous localportion succeeding the writing pulse might would optically separated.Consequently that x is ordinarily 2.0 or smaller.

[0068] Further, if the value y is excessively large, there is also adanger that an old mark might be inadequately crystallized duringoverwriting. Consequently x is ordinarily 2.0 or smaller.

[0069] It is preferable that the present invention is used in forming anamorphous mark by a pulse pattern of two or more writing pulses. A shortmark such as 3T mark (n=3 in nT mark) may be formed by a pulse patternof a single writing pulse. In the case of a single writing pulse 1,since only the bias power Pb or the erasure power Pe is irradiated afterthe writing pulse 1, the leading edge of the mark is relatively rapidlycooled. Otherwise in the case of two or more writing pulses, since thehigh-power writing pulse 2 is irradiated subsequently to a possibleoff-pulse succeeding the writing pulse 1, it would be difficult to coolthe mark. Consequently as proposed by the present invention, it isremarkably effective to make both xT pulse and yT pulse longer in timelength. It is preferable that the present invention is applied informing a mark having a time length equal to or larger than 4T mark (n=4in nT mark).

[0070] The pulse lengths of the writing pulses 1 through 9 of FIG. 1will now be described using FIG. 2.

[0071]FIG. 2 illustrates pulse lengths in a representative pulse patternto be used in the recording method of the present invention. The numberof writing pulses of FIG. 2 is m in total (m is a natural number), m=6as the illustrated example. Further, the individual writing pulses 1through 6 have respective pulse lengths α₁T through α₆T; subsequently tothe individual writing pulses, off-pulses having respective pulselengths β₁T through β₆T are respectively located. The total time lengthof the combination of m writing pulses and m off-pulses is nearlyequivalent to the mark length nT. Therefore the following formula issatisfied. $\begin{matrix}{{\sum\limits_{i = 1}^{m}\left( {\alpha_{i} + \beta_{i}} \right)} \approx n} & (3)\end{matrix}$

[0072] i is a natural number, α_(i) is a coefficient determining a timelength of the i-th writing pulse, and β_(i) is a coefficient determininga time length of the i-th off-pulse.$\sum\limits_{i = 1}^{m}\left( {\alpha_{i} + \beta_{i}} \right)$

[0073] represents the sum of total (α_(i)+_(i)) for i from 1 to m. Σ^(m)_(i=1)(α_(i)+β_(i)) should by no means be precisely equal to n,ordinarily nearly n−2 to n+2.

[0074] If the time length α₁T of the leading pulse (writing pulse 1) ofFIG. 2 is excessively large, the mark would be excessively heated sothat re-crystallization cannot be retarded even when both the off-pulsespreceding and succeeding the leading pulse are increased in time length,which would be failed to retard re-crystallization. Consequently thevalue α₁T is preferably 1.5T or smaller, more preferably 1.0T orsmaller, much more preferably 0.8T or smaller. Otherwise if the valueα₁T is excessively small, the temperature of the recording layer canonly inadequately rise. Consequently the value α₁T is preferably 0.2T orlarger, more preferably 0.3T or larger.

[0075] Further, the pulse length α₁T is set to preferably a valuesmaller than the second set time length xT so that the leading edge ofthe mark can be rapidly cooled.

[0076] Furthermore, if the time length α_(i)T (i is a natural numbersatisfying i≧2) of the second or succeeding writing pulse is excessivelylarge, it would be difficult to retard re-crystallization. Consequentlythe value α₁T is preferably 0.8T or smaller, more preferably 0.6T orsmaller. Otherwise if the value α₁T is excessively small, thetemperature of the recording layer can only inadequately rise.Consequently the value α₁T is preferably 0.2T or larger, more preferably0.3T or larger.

[0077] β_(m)(e.g., e β₆) appearing at the trailing end of the pattern isordinarily within a range of from 0T to 1.5T; by varying this timelength, it is possible to control the time length of an amorphous mark.If this value is excessively large, there would be a danger that an oldmark might be inadequately crystallized during overwriting.

[0078] In the present invention, the period of irradiation for each ofpulses (the writing pulse 2 or any writing pulse subsequent thereto)subsequent to the leading pulse (writing pulse 1), i.e. a time sectionduring which the high-power energy beam is irradiated, is set to a valueclose to 1T. Specifically this value is within a range of from 0.5T to1.5T. The term “period” is a time length from the rising of the writingpulse to the rising of the next writing pulse.

[0079] Namely, α_(i)+β_(i) (i is a natural number, i=2 through m−1) hasa value close to 1. Specifically this value is within a range of from0.5 to 1.5. If this value is far remote from 1, the following problemswould occur.

[0080] For forming an amorphous mark, it is essential that the value ofβ_(i) is somehow large to increase the cooling rate. Otherwise ifα_(i)+β_(i) is far smaller than 1, it is impossible to increase β_(i) toan adequate extent so that the cooling rate would become small, tendingto be a cause for re-crystallization of the mark. Reversely, if β_(i) islarge, α_(i) would be small so that the temperature of the recordinglayer is difficult to rise to an adequately high value.

[0081] In the meantime, the concept of increasing α_(i)+β_(i) (i is anatural number, i=2 through m−1) to a value far larger than 1 isdisclosed in Japanese Laid-Open Publication No. Hei 9-134525 (U.S. Pat.No. 5,732,062). However, assuming that α_(i)+β_(i) is fixed to a valueequal to or larger than 2, the length of the mark would be increased by2T as the number of the writing pulses (and the off-pulses associatedtherewith) is increased by 1. Therefore, in this known recording method,it is impossible to separately record two different kinds of marks, suchas nT mark and (n+1)T mark, whose difference is 1T.

[0082] Consequently, in the present invention, a writing pulse (andoff-pulses associated with the writing pulse) such that the value ofα_(i)+β_(i) is 1 is added to an nT-mark pulse pattern at any location toobtain an (n+1)T-mark pulse patter so that the difference between thetime length of nT mark and that of (n+l)T mark is 1T.

[0083] Namely, the value of α_(i)+β_(i) (i is a natural number, i=2through m−i) is non-uniform as different values such as 1 and 2 exist atrandom.

[0084] However, the values α_(i) and β_(i) are significantly influentialon the temperature distribution and temperature process in the recordinglayer so that the temperature or temperature varying process would varyconsiderably at each place where a different value α_(i) or β_(i)exists. Since not only the reachable temperature but also the size ofre-crystallized area after a meltdown, which area is related with atleast the cooling rate, are significantly influential on the temperaturedistribution of the recording layer, the mark would vary in shape in acomplicated manner depending on the various factors.

[0085] Under a particular recording condition, the values α_(i) andβ_(i) could be determined in such a manner that the difference betweennT mark length and (n+1)T mark length is 1T. But such values can beeffective only for a very limited recording condition so that it ishighly likely that the difference of mark length would shift far from 1Tunder a recording condition slightly different from the above-mentionedparticular recording condition. Such problem would occur due to thechange in luminescence distribution in the laser beam spot when therecording power and/or the recording apparatus have been changed.

[0086] This is true because the temperature distribution change of therecording layer with respect to the recording condition change dependson the value of α_(i)+β_(i). For example, since the mark shape varieswith the recording condition change in different manners respectivelyfor 1 and 2 as the values of α_(i)+β_(i), it would be difficult tocontrol the mark shape in the presence of the mixed values ofα_(i)+β_(i).

[0087] Whereas, in the present invention, if the value α_(i)+β_(i)(i isa natural number, i=2 through m−1) is close to 1, it is possible toincrease the mark longer by 1T, without largely changing the temperaturedistribution, by a single pulse to a succession of pulses at the centralportion of a mark forming pulse, so that such problem would hardlyoccur. Since the value of α_(i)+β_(i) is constant, the temperaturedistribution change due to the recording power change is the same forevery mark length.

[0088] According to the present invention, since re-crystallizationduring recording is retarded by rapidly cooling the leading edge of themark, it is unnecessary to vary the period of irradiation for the secondwriting pulse and those subsequent thereto. For this purpose, the periodof irradiation for the succeeding pulse (recording pulse 2 and thatsubsequent thereto), subsequent to the leading pulse (writing pulse 1)and having a time section during which the high-power energy beam isirradiated, has a value closed to 1T.

[0089] But if the mark is short, the temperature for the leading andtrailing edges of the mark varies somehow as another pulse is added tothe mark; therefore the value of α_(i)+β_(i) is occasionally preferableto be shifted off 1. Consequently, in the present invention, the valueof the pulse irradiation period with the high-power energy beam is 0.5Tor larger, preferably 0.8T or larger. And this value is 1.5T or smaller,preferably 1.2T or smaller.

[0090] The value of α_(m)+β_(m)at trailing end of the mark may exceed1.5 by a large extent so that the length of an amorphous mark can becontrolled by varying the length β_(m)T of the trailing end off-pulse.

[0091] Thus, even if recording is made in a recording medium that has areflective layer disposed contiguously to the phase-change recordinglayer with or without the medium of a protective layer, or is void of areflective layer or small in thickness, it is possible to preventre-crystallization of the mark during recording while keeping anadequate erasure ratio during irradiation of the erasure power. Alsorecording can be made without any risk of restricting the range ofeffective crystallization speeds. Further, since the influence ofre-crystallization is more remarkable during the mark length modulationrecording, the present invention is particularly useful when used duringthe mark length modulation recording.

[0092] The construction of the phase-change recording medium on whichrecording is made according to the present invention will now bedescribed; this description will start with the materials to be used inthe recording layer.

[0093] The present invention is particularly useful when applied to achange-phase recording medium in which a material tending to bere-crystallized are used in a recording layer. The recording layermaterial tending to be re-crystallized is exemplified by the followingcomposition, which contains an excessive amount of Sb as compared to anSub-Te eutectic composition.

[0094] Preferably the composition of the recording layer material is(Sb_(a)Te_(1−a))_(b)M_(1−b) (where 0.6<a<0.9, 0.7<b<1, and M is at leastone element selected from the group consisting of Ge, Ag, In, Ga, Zn,Sn, Si, Cu, Au, Pd, Pt, Pb, Cr, Co, O, S, Se, V, Nb, Ta).

[0095] The recording layer whose composition contains an excessiveamount of Sb as compared to an Sb—Te eutectic composition has a tendencyof being re-crystallized as compared to a recording layer of Ge₂Sb₂Te₄composition, which is ordinarily used as a recording layer of aphase-change recording medium. The reason why the phenomenon in which anamorphous mark tends to be re-crystallized is significant in therecording layer of this composition. But partly because crystal nucleiare generated in the recording layer of Ge₂Sb₂Te₅ composition, andpartly because crystallization progresses from the edge of the amorphousmark with respect to the crystalline area around the mark edge, thedifference in mechanism of re-crystallization could be considered as areason.

[0096] In a phase-change recording medium whose recording layer containsthis composition and in which the cooling effect is not remarkable, itis presumed that re-crystallization tends to occur the leasing edge ofthe mark, particularly at the transversely (perpendicularly to thetracking direction) central portion of the leading mark edge, and thenprogresses from there toward the trailing edge of the mark.

[0097] Since re-crystallization is retarded by rapidly cooling theleading edge of the mark, it is possible also retard re-crystallizationfrom progressing from the leading edge to the trailing edge of the mark.Therefore the cooling effect of the present invention is particularlysignificant when the present invention is applied to alow-cooling-effect phase-change recording medium. Namely, the presentinvention is particularly useful when recording is made on aphase-change recording medium that is devoid of a reflective layerdisposed contiguously to the recording layer with or without the mediumof a protective layer, or has a very thin reflective layer of 30 nm orsmaller in thickness.

[0098] Assuming that the phase-change recording medium is a mediumhaving two or more phase-change recording layers with another layersandwiched therebetween, the write/retrieve energy power beam has toreach also the farther phase-change recording layer, and therefore athick reflective layer cannot be located in the traveling path of thepower beam. As a consequence, for the nearer phase-change recordinglayer, the phase-change recording medium to be used in this example islimited to a medium that is devoid of a reflective layer disposedcontiguously to the recording layer with or without the medium of aprotective layer, or has very thin reflective layer of 30 nm or smallerin thickness. The present invention is particularly useful when used inmaking recording on the nearer phase-change recording layer with respectto the write/retrieve energy power beam.

[0099] Secondly the layer structure of the phase-change recording mediumto which the present invention is applied will now-be described withreference to FIG. 3. FIG. 3 schematically depicts a multi-layerstructure of a phase-change recording medium. The phase-change recordingmedium 10 of FIG. 3 comprises a reflective layer 10 a, a first recordingmedium part 1, a resin layer 10 e, a second recording medium part 2, anda substrate 10 i. The energy beam enters the recording medium 10upwardly from the substrate side, i.e. from the lower side.

[0100] The reflective layer 10 a reflects the energy beam to disperseheat incoming from a first recording layer 10 c via the protective layer10 b. The first recording medium 1 is composed of protective layers 10b, 10 d, and the first recording layer 10 c. The protective layers 10 b,10 d controls absorption of the energy beam to adjust the reflectance,and also controls heat radiation from the first recording layer 10 c toretard heat deformation of the first recording layer 10 c.

[0101] The first recording layer 10 c is composed of a phase-changematerial and varies in its optical characteristic as the crystallinestate is reversibly changed. The resin layer 10 e serves as a spacer toadjust the position of the recording layer 10 c to a focus length of theenergy beam.

[0102] Likewise, the second recording medium part 2 is composed ofprotective layers 10 f, 10 h, and a second recording layer 10 g. Theprotective layers 10 f, 10 h and the second recording layer 10 g areidentical in function with the protective layers 10 b, 10 d and thefirst recording layer 10 c, so repetition of description is omittedhere.

[0103] Further, the substrate 10 i has on its outer surface a recess.For the material of the substrate 10 i, resin such as polycarbonate,polyacrylic or polyolefin, or glass may be used. In the presentembodiment, since the light (power) beam enters the recording medium 10via the substrate 10 i, the substrate 10 i has to be transparent. Forthe material of the resin layer 10 e, the same material as that of thesubstrate 10 i may be used.

[0104] The material for the reflective layer 10 a will now be described.The reflective layer material is preferably high in reflectance and heatconductance. The reflective layer material high in reflectance and heatconductance is exemplified by an alloy containing Ag, Au, Al, or Cu as achief component. Among these elements, Ag is an element highest inreflectance and heat conductance.

[0105] For short wavelength light, since Au, Cu and Al tend to absorblight as compared to Ag, it is most preferable to use Ag whenshort-wavelength laser of 650 nm or less is used. Further, Ag isrelatively inexpensive as a sputtering target, stable in discharging,high in deposit speed, and stable in atmosphere.

[0106] In the presence of impurities contained, Ag, Al, Au, Cu would besomehow lowered in heat conductance and reflectance, and therefore theyare not preferable in that respect. But for the other respect, theycould be improved in stability and film surface flatness and thereforemay contain about 5 atom % or less of an impurity element, such as Cr,Mo, Mg, Zr, V, Ag, In, Ga, Zn, Sn, Si, Cu, Au, Al, Pd, Pt, Pb, Ta, Ni,Co, O, Se, Nb, Ti, N. The thickness of the reflective layer may beordinarily within a range of from 50 to 200 nm for good result. If thereflective layer is too thin, adequate reflectance and radiating effectcannot be achieved. Otherwise if it is too thick, the reflective layeris not preferable in view of film strain (membrane strain),manufacturing period and cost.

[0107] The phase-change recording medium of the illustrated embodimentis void of any reflective layer between the protective layer 10 f andthe resin layer 10 e. Because it is necessary to permit the light beamto reach the recording layer 10 c, a thick reflective layer cannot belocated in the traveling path of the light beam. But a very thinreflective layer of 30 nm or less, preferably 20 nm or less, inthickness may be located.

[0108] With this layer structure, the recording method of the presentinvention is particularly useful in recording to the second recordingmedium part 2. This is true because the second recording medium part 2requires high transmission factor and, for that purpose, is void of anyreflective layer or has only a very thin reflective layer so that heatradiating effect is small.

[0109] The material of the protective layers 10 b, 10 d, 10 f, 10 h willnow be described. The material of these protective layers 10 b, 10 d, 10f, 10 h is determined in view of refractive index, heat conductance,chemical stability, mechanical strength, tight contact. For theprotective layer material, in general, a metal high in transparency andmelting point or oxide, sulfide, nitride of semiconductor, or fluorideas of Ca, Mg, Li may be used. These oxide, sulfide, nitride and fluorideshould by no means in the form of a chemical quantitative composition;it is also effective to modify in composition and use differentcomposition in a composite form.

[0110] In view of repeating recording characteristic, the material ofthe protective layers 10 b, 10 d, 10 f, 10 h is preferably a dielectricmixture. More specifically, the dielectric mixture is exemplified by amixture of ZnS or rare earth sulfide and a heat-resistant chemicalcompound such as oxide, nitride or carbide. For example, a mixture ofZnS and SiO₂ is used for the protective layer material of many ofcommercially available phase-change optical discs. The film density ofthese protective layers 10 b, 10 d, 10 f, 10 h is preferably 80% or morein bulk state in view of mechanical strength.

[0111] Further, if the dielectric layer (protective layer) is less than10 nm in thickness, it is impossible to adequately prevent the substrate10 i and the recording layers (recording layers 10 c, 10 g) from beingdeformed so that the protective layers 10 b, 10 d, 10 f, 10 h do nottend to perform the original functions. If the dielectric layerthickness exceeds 500 nm, the curving of the dielectric layer whenplaced on the substrate 10 i is proportional to the film thickness,which would be a cause for possible cracks.

[0112] Particularly the lower protective layers 10 d, 1010 h mustrespectively retard the substrate deformation due to heat and, for thispurpose, is preferably 70 nm or more in thickness. This is because, ifthe protective layer thickness is less than 70 nm, micro substratedeformations would accumulate during repeated overwriting so thatretrieving light would scatter to increase noises.

[0113] The upper limit of thickness of the protective layers 10 b, 10 d,10 f, 10 h is practically nearly 200 nm in view of depositing time. Thisis because, if the thickness is larger than 200 nm, the groove geometryas viewed on the recording layer surface would change. Namely, the depthof the groove would be smaller than that in the intended shape on thesurface of the substrate 10 i, and the width of the groove also would benarrower than that in the intended shape on the surface of the substrate10 i. Further, the protective layer thickness is more preferably 150 nmor less.

[0114] Further, the thickness of the first and second recording layers10 c, 10 g is preferably within a range of from 3nm to 20 nm, morepreferably from 5 to 10 nm. This is because, if the thickness of thefirst and second recording layers 10 c, 10 g is thin, an adequatecontract between reflectance in crystalline state and that in amorphousstate would be difficult to achieve and initial crystallization would bedifficult. Otherwise if it is too thick, the first and second recordinglayers 10 c, 10 g tend to decrease in transmittance.

[0115] The thicknesses of the first and second recording layers 10 c, 10g and the protective layers 10 b, 10 d, 10 f, 10 h are respectivelyselected in such a manner that the absorption efficiency of laser lightis improved, and a contrast in amplitude of a recording signal betweenthe recorded state and the unrecorded state would become large, withconsidering the restriction in view of mechanical strength andreliability, and the interference effect.

[0116] The protective layers 10 b, 10 d, 10 f, 10 h, the first andsecond layers 10 c, log and the reflective layer 10 a are formedordinarily using the sputtering method. It is desirable to performdepositing for these various layers on a sputtering apparatus (in-lineapparatus), with the recording layer target, the protective layer targetand the reflective layer target, if necessary, being placed in a commonvacuum chamber for the purpose of preventing interlayer oxidation andcontamination and also attaining an excellent rate of production.

[0117] The present invention should by no means be limited to theforegoing embodiment and variations, and various other changes ormodifications may be suggested without departing from the gist of theinvention.

[0118] The energy beam to be used in recording also should by no meanslimited to laser light, and an alternative device may be used.

EXAMPLE

[0119] A ZnS—SiO₂ lower protective layer (100 nm thick), G_(e8)Sb₆₅Te₂₇recording layer (7 nm thick), and ZnS—SiO₂ upper protective layer (160nm thick) were deposited on a 0.6 mm thickness polycarbonate substratehaving a guide groove(s) by sputtering, whereupon the upper protectivelayer was coated with a UV-curable resin protective coating. The guidegroove of the substrate is 33 nm in depth, 348 nm in width, and 74 μm inpitch.

[0120] Using an optical disc evaluating apparatus having an opticalsystem which is 0.6 in NA (Numerical Aperture) and 635 nm in laserwavelength, the resulting optical disc was crystallized to initialize byirradiating DC (direct current) light whose output power is 3 mW at a1.8 m/s linear velocity.

[0121] Subsequently the following measurement was carried out. Allsignals were recorded in the guide groove.

[0122] First of all, the erasure ratio indicating a ratio of reinstatingcrystalline from amorphous was measured under the following conditions:A single pattern signal of approximately 10T in mark length was recordedin the guide groove at a linear velocity of 3.8 m/s, with the output ofa reference clock period T of 38.2 ns (1/26.16 MHz), a writing powerPw=11 mW, an erasure power Pe=1 mW, a bias power Pb=1 mW, a retrievepower Pr=1 mW. The pulse pattern of FIG. 1 was used to determine:α_(i)(i≧1)=0.3, β_(i)(i≧2)=0.7, x=0.7, y=0.

[0123] The present inventor(s) measured the erasure ratio by irradiatingDC light having a predetermined erasure power Pe to the thus recordedsignal at a 3.8 m/s linear velocity. The results are shown in FIG. 4.

[0124]FIG. 4 is a graph illustrating the transition of the erasure ratiowith respect to the erasure power Pe. As shown in FIG. 4, the erasureratios of nearly 25 dB were obtained with respect to the erasure powerPe of 3.5 mW. The C/N ratio of the unerased signal was 52 dB.

[0125] Then, recording was made on this optical disc, and thenretrieving was made, whereupon the jitter was measured. Under basicallythe same recording conditions as those during recording, namely, theerasure power Pe of 3.5 mW, with respect to which value the mostpreferable erasure ratio was obtained as mentioned above, the writepower Pw=11 mW; the same single pattern was recorded by varying thevalue of x with respect to α_(i)(i≧1)=0.3, β_(i)(i≧2)=0.7y=0 using thepulse pattern of FIG. 1. The thus recorded signal was retrieved at a 3.8m/s linear velocity with the retrieve power Pr=0.8 mW, whereupon thejitter was measured. The results are shown in FIG. 5.

[0126]FIG. 5 is a graph illustrating a relation between x of thesucceeding low-power pulse irradiation section and the jitter. It turnsout from the graph of FIG. 5 that if the value of x was small, thejitter is large, and if the value of x was large, the jitter is small.From the retrieved waveform when x was small, it was observed that theleading edge of the mark was crystallized.

[0127] Subsequently, the same measurement was carried out by varying thevalue of y with x=0.7. The results are shown in FIG. 6.

[0128]FIG. 6 is a graph illustrating a relation between y of thepreceding low-power pulse irradiation section and the jitter. If thevalue of y became large, the jitter became small. Namely, it isunderstood that both x and y were effective to reduce the jitter.

[0129]FIG. 7 is a graph illustrating between the value of x+0.7 *y andthe jitter. As shown in FIG. 7, it turns out from the experiments of thepresent inventor(s) that the value of x+0.7 *y were excellentlycorrelated with the jitter. Namely, when (x+0.7*y)≧0.95, the jitter was10 ns or less; and when (x+0.7 *y)≧1.3, the jitter was 6.5 ns or less.

[0130] Further, the transmission factor was approximately 55% when therecording layer was in amorphous state, and approximately 47% when therecording layer was in crystalline state.

[0131] The transmission factor was obtained in the following manner.Laser light of 635 nm wavelength was penetrated through a 0.6 mm thickpolycarbonate substrate, and then a 200 nm thick Al_(99.5)Ta_(0.5) filmwas retrieved, whereupon the reflected light amount I₁ was measured. Thesame laser light was penetrated through the optical disc of the presentembodiment, and then a 200 nm thick Al_(99.5)Ta_(0.5) film wasretrieved, whereupon the reflected light amount I₂ was measured. Thetransmission factor was obtained by the formula (I₂/I₁)^(½)*100(%).

[0132] The results indicate that some in-disc characteristicdistribution presumably due to the small-thickness and tending-to-curvesubstrate and the non-uniform protective coating permitting transmissionof light.

[0133] It is understood that it is possible to form and erase anamorphous mark easily even in the absence of a reflective layer bydetermining x and y within respective particular range.

What is claimed is:
 1. A recording method for a phase-change recordingmedium having a phase-change recording layer in which amorphous markseach having a time length nT (T is the data reference clock period, andn is a natural number equal to or larger than 4) are formed byalternately irradiating the recording medium at least with a high-powerenergy beam having a relatively high power value and a low-power energybeam having a relatively low power value, said method comprising thesteps of: (a-1) irradiating the low-power energy beam to the recordingmedium for a first set time yT (y is a natural number equal to or largerthan 0) as a preceding low-power pulse irradiation step immediatelybefore a prospective leading pulse that is a time section where thehigh-power beam is irradiated for the first time, and (a-2) irradiatingthe low-power energy beam to the recording medium for a second set timexT (x is a natural number larger than 0) as a succeeding low-power pulseirradiation step immediately after said prospective leading pulse, x andy having a relation expressed by the following formula (A) 0.95≦x+0.7*y≦2.5   (A) where * is an arithmetic symbol representing amultiplication; and (b) irradiating the high-power energy beam to therecording medium in such a manner that a period of irradiation forpulses subsequent to said leading pulse is in a range of from 0.5T to1.5T.
 2. A recording method for a phase-change recording mediumaccording to claim 1, wherein y is a natural number larger than
 0. 3. Arecording method for a phase-change recording medium according to claim1, wherein 1.3≦x+0.7*y≦2.0.
 4. A recording method for a phase-changerecording medium according to claim 2, wherein 1.3≦x+0.7*y≦2.0.
 5. Arecording method for a phase-change recording medium according to claim1, wherein said phase-change recording layer comprises an alloycomposition having an excessive amount of Sb as compared to an Sb—Teeutectic composition.
 6. A recording method for a phase-change recordingmedium according to claim 2, wherein said phase-change recording layercomprises an alloy composition having an excessive amount of Sb ascompared to an Sb—Te eutectic composition.
 7. A recording method for aphase-change recording medium according to claim 5, wherein said alloycomposition of said phase-change recording layer comprises a chiefcomponent expressed by the following formula:(Sb_(a)Te_(1−a))_(b)M_(1−b)   (B) where a is a real number in a range of0.6<a<0.9, b is a real number in a range of 0.7<b<l, and M is at leastone elements selected from the group consisting of Ge, Ag, In, Ga, Zn,Sn, Si, Cu, Au, Pd, Pt, Pb, Cr, Co, O, S, Se, V, Nb, Ta.
 8. A recordingmethod for a phase-change recording medium according to claim 6, whereinsaid alloy composition of said phase-change recording layer comprises achief component expressed by the following formula:(Sb_(a)Te_(1−a))_(b)M_(1−b)   (B) where a is a real number in a range of0.6<a<0.9, b is a real number in a range of 0.7<b<1, and M is at leastone elements selected from the group consisting of Ge, Ag, In, Ga, Zn,Sn, Si, Cu, Au, Pd, Pt, Pb, Cr, Co, O, S, Se, V, Nb, Ta.
 9. A recordingmethod for a phase-change recording medium according to claim 1, whereinthe time length of the irradiation of said high-power energy beam forsaid leading pulse is shorter than said second set time xT.
 10. Arecording method for a phase-change recording medium according to claim1, wherein said phase-change recording medium is a medium devoid of areflective layer that is disposed contiguously to said phase-changerecording layer or with the medium of a protective layer to saidphase-change recording layer.
 11. A recording method for a phase-changerecording medium according to claim 1, wherein said phase-changerecording medium is a medium having a reflective layer of 30 nm orsmaller in thickness and disposed contiguously to said phase-changerecording layer or with the medium of a protective layer to saidphase-change recording layer.
 12. A recording method for a phase-changerecording medium according to claim 1, wherein said phase-changerecording medium is a multi-layer recording medium having two or morephase-change recording layers sandwiching an intermediate layertherebetween.